This invention relates to lighting control systems and, more particularly, to lighting control systems which operate automatically to provide illumination when a room is occupied.
Up to 50% of the electric energy costs for a commercial building are for lighting. Much of this cost is wasted, either because the area illuminated in the building is unoccupied or is sufficiently illuminated during daylight hours by sunlight passing through windows. Some static methods have been used to improve the situation. These include removing lamps from certain fixtures and using lamps which are more efficient than conventional incandescent and fluorescent lights. However, in more recent years automatic lighting control systems have been used.
These automatic systems adjust the amount of electrically generated illumination in response to the ambient sunlight available and also automatically turn off the lights when the room or area is unoccupied.
A simple form of automated control employs computers or timers to turn the lights on and off at preset times. This occurs so that after working hours the lights are not accidentally left on. The problem with such a system is that frequently it is necessary to have the lights on at night for maintenance and cleaning personnel, as well as regular employees who must work late.
A more sophisticated system uses photodiodes to control the lighting system based on available ambient lighting. Such a system can turn off unneeded lights or dim their output when sufficient sunlight is available. An example of such a system is disclosed in U.S. Pat. No. 4,383,288 of Hess, et al.
With photodetector type lighting control systems, there is still wasted energy because lights are not turned off in unoccupied areas. One way of correcting this is by incorporating occupancy detectors into the control system. Such detectors may operate by utilizing ultrasonic or infrared detectors. These devices use shifts in received ultrasonic or infrared energy to indicated movement of a person into and within the area. If no movement is detected within a particular period of time, the system turns off the lights in the area.
Commercial examples of ultrasonic control systems are sold under the tradename Enertron UD and Light-O-Matic Model 01-071. These devices, however are subject to false triggering due to noise vibrations unrelated to the occupancy of the room. Thus they are inaccurate and highly unreliable.
Lighting systems controlled by passive infrared sensors are much less sensitive to extraneous signals than ultrasonic models. Commercial versions of these systems are sold by United technologies under the name Infracon. This type of device, however, can only cover an arc of about 60.degree. because passive infrared detectors are characterized by a lambertian distribution of sensitivity. In particular the sensitivity decreases as the cosine of the angle from the optical axis. Thus at an angle of 30.degree. from the optical axis the sensitivity is only half what it is at the center. As a result the signal-to-noise ratio is decreased at the edges of a 60.degree. arc and the effectiveness of the detector is lessened.
One way to improve the effective arc of an infrared detector is to employ a lens to direct the heat energy from a wider angle into the effective area of the lens. Such a system is sold under the name LightWatch by Colorado Electro-Optics. This device uses two Fresnel lenses and has an effective arc of about 90.degree..
In a typical room the most useful detector would be a passive infrared type with an effective arc approaching at least 180.degree.. In such an arrangement the detector could be located near one wall and could scan the entire room for heat changes that indicate occupancy of the room. However, there are no known prior art detectors with this capability.